There is considerable interest in reducing the length of fluorescent lamps while maintaining reasonable levels of light output and efficacy. Prior art fluorescent lamps were generally operated from line current sources in conjunction with inductive and/or resistive auxiliary ballast components. Recently developed electronic power supply and current-limiting circuits are available as replacements for heavy, expensive prior art inductors and transformers, and have made the operation of short fluorescent lamp tubes at high current and low voltage economically feasible. In order to obtain reasonable efficacy from an arc discharge column at high current, it is necessary to operate with a fill gas which comprises a large percentage of light, inert gas atoms, i.e., neon; at low pressure, i.e., below 3 torr.
Conventional filament electrodes perform poorly at ac currents in excess of 2 amperes. Filament electrodes are characterized by a cathode spot which, at high currents, becomes very intense and tends to cause rapid evaporation and loss of active emission material. At the same time, the electrode voltage drop rises with increasing current as the cathode spot is depleted of emission material. Electrode damage and high voltage drop is a particularly severe problem in a low pressure neon-mercury vapor fill gas inasmuch as neon has a small atomic cross section and thus allows a more rapid diffusion loss of evaporated cathode material and mercury ions in the discharge.
It is well known that hollow cathodes present an attractive alternative to filament cathodes in high current fluorescent lamps. Properly designed hollow cathodes emit diffusly from a relatively large area; that is, they do not emit from a small hot cathode spot; have a low cathode fall voltage; and tend to contain evaporated emission material within a hollow cavity. U.S. Pat. No. 3,883,764 to Peter D. Johnson and John M. Anderson describes a cylindrical hollow cathode which surrounds a conventional filament cathode. The filament aids in starting the discharge and dispenses emission material to the inner surface of the cylindrical hollow cathode.